required root for this procedure
first you need to backup EFS folder from 4.1.2 which stores the IMEI number and serial number . After upgrading to 4.3, you cannot downgrade
Here EFSv1 change to EFSv2 ,so downgrading will damage the IMEI . you can downgrade if you have the 4.1.2 EFS backup .

Yes- definitely stay with 4.1.2. I put 4.3 on my S3 and it hasn't been the same phone since. Samsung should be fined heavily for this lash up, as should the carriers for supposedly checking 4.3 for the best customer experience. Bad joke.